/Engine/Source/Runtime/Slate/Public/Framework/Application/SlateApplication.h
Replace theIPlatformTextFieldimplementation with a custom one. The implementation used by default is platform specific. If you need to change the default behavior for virtual keyboard, this method is for you.
void OverridePlatformTextField ( [TUniquePtr](API\Runtime\Core\Templates\TUniquePtr)< [IPlatformTextField](API\Runtime\Slate\Framework\Application\IPlatformTextField) > PlatformTextField )